This invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
Field of the Invention
This invention relates to a process of post-treating metal coated substrates, and to the composition for treating metal coated substrates to provide color recognition and improve adhesion-bonding, abrasion, and corrosion-resistant properties of the coated substrates. More specifically, this invention relates to a novel composition, and to the process of using the composition to post-treat metal coated substrates. The composition comprises an acidic aqueous solution comprising effective amounts of at least one water-soluble trivalent chromium compound, an alkali metal hexafluorozirconate, at least one tetrafluoroborate and/or hexafluorosilicate, at least one water soluble zinc compound and effective amounts of water-soluble thickeners.
Current high-performance post treatments for metal coated substrates are based on hexavalent chromium chemistry. Hexavalent chromium is highly toxic and a known carcinogen. As a result, the solutions used to deposit post-treatment coatings and the coating, per se are toxic. These coatings do, however, yield outstanding paint adhesion and corrosion resistance to the base metal. Typically, post-treatments or coatings are deposited onto the metal at ambient temperatures and usually applied by immersion or spray processes. Post treatments are usually specified by the military or commercial specifications that govern each metal coating being treated. As such, there is not a unique xe2x80x9cpost treatmentxe2x80x9d specification for all metal coatings as there is for xe2x80x9cconversion coatedxe2x80x9d aluminum and its alloys.
Further, environmental laws, executive orders, and local occupational, safety, and health (OSH) regulations are driving military and commercial users in the search for chromate-free post treatments. In the case of these coated metals, the metal substrates and the coatings per se are relatively non-toxic. With the addition of a chromate post treatment, however, these coatings become toxic. In addition, the use of chromate post treatments is becoming more expensive as regulations tighten and costs are becoming prohibitive with restrictions imposed by the EPA. Moreover, certain processes like spraying chromate coating solutions are forbidden at some facilities due to OSH risk, thereby forcing the use of less-than-optimum solutions. Thus, while existing chromate post treatments are outstanding in their technical performance in that they provide enhanced corrosion protection and adhesion bonding e.g. adhesion for paint and other coatings at a low application cost, from a life-cycle cost, environmental, and OSH perspective, chromate coatings are detrimental for people and the environment.
This invention relates to trivalent chromium post treatment (TCP) compositions and to the processes for providing color recognition and improved adhesion and corrosion resistant properties of metal coated substrates. These coatings and processes are generally known as xe2x80x9cpost-treatmentsxe2x80x9d. Post-treatments are coatings in contact with the metal being treated after formation of the initial coating and therefore the post-treatments normally have no direct contact with the underlying substrate, except possibly through some pores in the outer metal coating.
Specifically, this invention relates to a composition and to the process of using said composition to post-treat metal coated substrates at ambient temperatures or higher e.g. temperatures ranging up to about 200xc2x0 F. More specifically, this invention relates to compositions for post-treating metal coatings to provide color recognition, and to improve the corrosion-resistance, and adhesion bonding properties e.g. paint adhesion of the coatings. The composition comprises an acidic aqueous solution having a pH ranging from about 2.5 to 5.5 and preferably 3.7 to 4.0, and comprises, per liter of said solution, from about 0.01 to 22 grams of a water-soluble trivalent chromium compound, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.0 to 12 grams and preferably 0.01 to 1.2 and more preferably 0.12 to 0.24 grams of at least one fluoro-compound selected from the group consisting of an alkali metal tetrafluoroborate, an alkali metal hexafluorosilicate and various combinations thereof, from about 0.001 to 10 grams per liter of at least one zinc compound, preferably divalent zinc salts, from about 0.0 to 10 grams per liter, preferably 0.5 to 1.5 grams of at least one water-soluble thickener and about 0.0 to 10 grams and preferably 0.5 to 1.5 grams per liter of at least one surfactant or wetting agent.
Compositions and processes based on trivalent chromium offer excellent technical performance compared to standard chromate-based coatings. However, a shortcoming of these compositions is the lack of a significant color change in the as-deposited metal coating, especially when used as a post treatment for aluminum-based sacrificial coatings. Chromate-based coatings of this type typically have an iridescent gold to brown color that is easily identified by processors, quality control personnel and other users in the field. A gold color on aluminum coated components generally means that a chromate post treatment is present and the color is useful for this type of quality control irrespective of technical coating performance such as resistance to corrosion or paint adhesion. TCP coatings typically have a light bluish to purplish to tan color, depending on the process conditions, that is very difficult to see in mixed light such as inside a production facility, at a repair depot or manufacturing plant. TCP coatings on some surfaces such as Ion Vapor Deposited (IVD) Aluminum is virtually colorless. Another shortcoming of these coatings is the relative performance of the TCP coatings as post treatments. As previously disclosed, TCP coatings yield about the same corrosion resistance as chromate post treatments on a variety of sacrificial coatings like cadmium, IVD aluminum, and zinc alloys such as zinc-nickel and tin-zinc. However, it would be desirable to have post treatment coatings that perform better than the presently known chromate post treatments.
It is therefore an object of this invention to provide an acidic aqueous solution comprising effective amounts of a trivalent chromium compound, an alkali metal hexafluorozirconate, a tetrafluoroborate and/or hexafluorosilicate and a divalent zinc compound for post-treating metal coated substrates to provide color recognition and identification, and to improve the adhesion bonding and corrosion resistance properties.
It is another object of this invention to provide a stable acidic aqueous solution having a pH ranging from about 2.5 to 5.5 containing a trivalent chromium salt, a hexafluorozirconate and a divalent zinc salt for post-treating metal coatings.
It is a further object of this invention to provide a stable acidic aqueous solution containing trivalent chromium having a pH ranging from about 3.7 to 4.0 for treating metal coated substrates at about room temperature wherein said solution contains substantially no hexavalent chromium.
These and other objects of the invention will become apparent by a further and more detailed description of the invention.
This invention relates to compositions and processes for preparing corrosion-resistant coatings on sacrificial coatings such as cadmium, zinc, IVD aluminum, Alumiplate, zinc-nickel and tin-zinc. The process comprises treating said metal coated substrates with an acidic aqueous solution containing a trivalent chromium salt basic, an alkali metal zirconate such as potassium hexafluorozirconate, a cellulose-based thickener and an alkali metal compound such as a potassium tetrafluoroborate or hexafluorosilicate stabilizer. The composition must also contain divalent zinc-based compounds to impart color change or color recognition to the as-produced coating. In most cases, the zinc-based compounds also increases the corrosion resistance imparted by the post-treatment coating to the sacrificial coatings. The amount of the zinc compound can be varied to adjust the amount of color that is imparted to the coating, from as little as 0.001 grams per liter up to 10 grams per liter e.g. 0.1 to 5.0 grams per liter.
After preparing the sacrificial coating by standard techniques, the composition of this invention can be used at room temperature and applied to the substrate via immersion, spray or wipe-on techniques similar to those used for conventional chromate post-treatments. Solution dwell time is 1 to 15 minutes. Dwell time can be increased to up to 30 minutes with increased corrosion resistance which may also decrease adhesion. A 10-minute dwell time yields an optimum film for color change, paint adhesion, and corrosion resistance. In this invention, the 1 to 15 minute dwell time yields appreciable color change to the as-deposited coating that ranges from light brown to dark bronze to gray-blue to bright cobalt blue depending on the composition of the sacrificial coating treated. In addition, the zinc compounds that yield color change in the as-deposited coating also improves the corrosion resistance over previously prepared TCP post treatments for sacrificial coatings. In some cases, for example on IVD aluminum, the corrosion resistance improvement approaches 100% over the standard chromate and TCP post-treatments.
Specifically, this invention relates an acidic aqueous solution having a pH ranging from about 2.5 to 5.5, and preferably from about 3.7 to 4.0 and to the use of said solution for post-treating metal coated substrates to provide color recognition and identification, and to improve the adhesive bonding, abrasion, and corrosion-resistance properties of the metal coatings. The compositions of this invention comprise, per liter of solution, from about 0.01 to 22 grams and preferably from about 0.01 to 10 grams e.g. 5 to 7 grams of at least one water soluble trivalent chromium compound e.g. trivalent chromium sulfate, about 0.01 to 12 grams and preferably about 0.01 to 8.0 grams e.g. 6 to 8 grams of at least one alkali metal hexafluorozirconate, about 0.0 to 12 grams and preferably from about 0.01 to 1.2 grams e.g. 0.12 to 0.24 grams of at least one alkali metal tetrafluoroborate and/or an alkali metal hexafluorosilicate and mixtures thereof at various ratio, from about 0.001 to 10 grams e.g. 0.1 to 5.0 grams of at least one divalent zinc compound e.g. zinc sulfate, an effective amount ranging from 0.0 to 10 grams e.g. from 0.5 to 1.5 grams of a water soluble thickener and from 0 to 10 grams of a water soluble surfactant.
In some processes, depending on the physical character of the metal coated substrate, the addition of a water soluble thickener to the solution aids in optimum film formation during spray and wipe-on applications by slowing down solution evaporation. This also mitigates the formation of powdery deposits that degrade paint adhesion. In addition, the use of thickeners aid in proper film formation during large area applications and mitigates the diluent effect of rinse water remaining on the substrate during processing from previous steps of the process. This feature yields films that have no streaks and are better in coloration and corrosion protection. The water soluble thickeners such as cellulose compounds are present in the solution in amounts ranging from about 0 to 10 grams per liter and preferably from 0.5 to 1.5 e.g. about 1.0 gram per liter of the aqueous solution. Depending on the character of the coated substrate, an effective but small amount of at least one water-soluble surfactant can be added to the acidic solution in amounts ranging from about 0 to 10 grams per liter and preferably from 0.5 to 1.5 grams e.g. 1.0 gram per liter of the acidic solution. These surfactants are known in the art of aqueous solutions and include organic compounds selected from the group consisting of non-ionic, cationic and anionic surfactants.
The chromium compound is added as a water-soluble trivalent chromium compound, preferably as a trivalent chromium salt. Although the resultant coating is rinsed with water, it is desirable to use chromium salts that provide anions not as corrosive as the chlorides. Preferably, these anions are selected from the group consisting of nitrates, sulphates, phosphates and acetates. Specifically, in formulating the acidic aqueous solutions of this invention, the chromium salt is added conveniently to the solution in its water soluble form wherein the valence of the chromium is plus 3. Some preferred chromium compounds are added to the solution in the form of Cr2(SO4)3, (NH4)Cr (SO4)2, Cr(OH)SO4, Cr2O3, or KCr(SO4)2 including various mixtures of these compounds. The preferred trivalent chromium concentration is within the range of about 5 to 7 grams or 6.0 grams per liter of the aqueous solution. It has been found that particularly good results are obtained when the trivalent chromium compound is present in solution in this preferred range. The preferred alkali metal fluorozirconate addition to the solution ranges from about 0.01 to 8.0 grams or 6.0 to 8.0 grams per liter. The zinc compound is preferably a divalent zinc salt such as zinc sulfate. The post-treatment of the metal coating can be carried out at ambient temperatures including temperatures of the solution or at higher temperatures e.g. temperatures ranging up to about 200xc2x0 F. Room temperature treatment is preferred in that this eliminates the necessity for heating equipment. The coating may be air dried by any of the methods known in the art, for example, oven drying, forced air drying, exposure to infra-red lamps, and the like.
There are key advantages in the use of the compositions and processes of this invention. One is the readily visible color change and recognition or identification imparted to the as-deposited coating compared to previously disclosed sacrificial coatings. TCP as previously disclosed causes a practical color change on zinc-nickel, but not on the other coatings such as IVD aluminum, Alumiplate, cadmium, zinc and tin-zinc.
The second advantage of the improvement is the increase in corrosion resistance for sacrificial coatings treated with the composition of this invention. Table C describes several examples of the corrosion performance of the disclosed invention and its relationship to sacrificial coatings treated with standard chromate post-treatments and previously disclosed versions of TCP post-treatments. As shown, the disclosed invention provides superior corrosion performance compared to no post-treatment to a previously disclosed TCP post-treatment and most importantly, to a standard chromate post-treatment. This is evident on four different types of IVD Aluminum coatings. The Zinc2+ cation or divalent compound can be supplied by any chemical source that dissolves in water and is compatible with the other components in the composition. Salts that are preferred include zinc acetate, zinc telluride, zinc tetrafluoroborate, and zinc sulfate. The composition, though typically used at ambient conditions, may be used at elevated temperature to increase reaction rate.